misc: dell-laptop should depend on POWER_SUPPLY

dell-laptop makes use of the power supply class information to choose
which backlight interface to change. Add a depends on it.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>